1. Field of the Invention
The present invention relates to an apparatus for measuring a superior-and-inferior-limb blood-pressure index of a living subject (e.g., a ratio of an inferior-limb blood pressure to a superior-limb blood pressure, or a ratio of a superior-limb blood pressure to an inferior-limb blood pressure).
2. Related Art Statement
Arteriosclerotic cardiovascular disease is one of major death causes of aged women, e.g., not younger than sixty-five-year old, but there is no easy method of finding latent arteriosclerosis. However, it is known that since measurement of superior-and-inferior-limb blood-pressure index can be used as a simple and repeatable method to find inferior-limb arterial disease and can be used to check quickly and easily the condition of whole cardiovascular system, the index is useful to identify individuals who need special treatments to reduce their death rate or incidence rate.
Generally, the above-mentioned superior-and-inferior-limb blood-pressure (xe2x80x9cBPxe2x80x9d) index is obtained as the ratio of a systolic blood pressure of an ankle as the inferior-limb to a systolic blood pressure of an upper arm as the superior-limb, that is, ankle/upper-arm BP index (abbreviated to xe2x80x9cABIxe2x80x9d). If the measured ankle/upper-arm BP index of a living subject is smaller than a prescribed value, e.g., about 0.9, abnormality may be diagnosed on the subject. Thus, a small change of the systolic blood pressure of the inferior limb such as ankle or a small change of the systolic blood pressure of the superior limb such as upper arm largely influences the diagnosis made on the subject. On the other hand, since blood pressure of each living subject may change in a short time, a conventional super-and-inferior-limb BP index measuring apparatus simultaneously starts increasing respective pressures of two cuffs respectively wound around the superior and inferior limbs to measure respective systolic BP values of the superior and inferior limbs.
However, even if the respective increasing of respective pressures of the two cuffs may be simultaneously started, there exist a certain amount of difference between the respective systolic BP values of the superior and inferior limbs. Therefore, there exists a certain time difference between respective times when the respective systolic BP values of the superior and inferior limbs are determined. The time difference corresponds to a few heartbeats of the subject. Meanwhile, the blood pressure of the subject contains a respiratory change corresponding to his or her respiration period. Thus, the blood pressure of the subject may change between the respective times of determination of the respective systolic BP values of the superior and inferior limbs. Thus, the superior-and-inferior-limb BP index determined by the conventional apparatus cannot enjoy a sufficiently high accuracy.
It is therefore an object of the present invention to provide a superior-and-inferior-limb blood-pressure index measuring apparatus which can measure a highly accurate superior-and-inferior blood-pressure index.
The Inventors have carried out extensive studies and found that if blood-pressure-relating information changing with blood pressure is iteratively obtained when inferior-limb and superior-limb blood pressures (systolic, mean, or diastolic blood pressures) are measured, a change of the blood pressure between respective times of measurement of the inferior-limb and superior-limb blood pressures is determined based on the iteratively obtained pieces of blood-pressure-relating information, and one of the inferior-limb and superior-limb blood pressures is corrected based on the thus determined blood-pressure change, then a highly accurate superior-and-inferior-limb blood-pressure index is obtained. The present invention has been developed based on this finding.
The above object has been achieved by the present invention. According to the present invention, there is provided an apparatus for measuring a superior-and-inferior-limb blood-pressure index of a living subject, comprising a first blood-pressure measuring device which includes at least one first inflatable cuff adapted to be wound around an inferior limb of the subject and measures a first blood pressure of the inferior limb; a second blood-pressure measuring device which includes a second inflatable cuff adapted to be wound around a superior limb of the subject and measures a second blood pressure of the superior limb; a blood-pressure-relating-information obtaining device for iteratively obtaining, from the subject, a piece of blood-pressure-relating information changing in relation with a change of a blood pressure of the subject; a blood-pressure-change-value determining means for determining a change value of the blood pressure of the subject between a first time when the first blood pressure is measured by the first blood-pressure measuring device and a second time when the second blood pressure is measured by the second blood-pressure measuring device, based on a first piece of blood-pressure-relating information obtained by the blood-pressure-relating-information obtaining device at the first time and a second piece of blood-pressure-relating information obtained by the blood-pressure-relating-information obtaining device at the second time; a corrected-blood-pressure determining means for determining, based on the change value determined by the blood-pressure-change-value determining means, one of the first and second blood pressures to a corrected blood pressure that would have been measured at one of the first and second times that corresponds to the other of the first and second blood pressures; and a blood-pressure-index determining means for determining the superior-and-inferior-limb blood-pressure index, based on the corrected blood pressure determined by the corrected-blood-pressure determining means and said other of the first and second blood pressures that has not been corrected by the corrected-blood-pressure determining means.
According to this invention, the blood-pressure-change-value determining means determines the change value of the blood pressure of the subject between the first time when the first blood pressure is measured by the first blood-pressure measuring device and the second time when the second blood pressure is measured by the second blood-pressure measuring device, based on the first piece of blood-pressure-relating information obtained by the blood-pressure-relating-information obtaining device at the first time and the second piece of blood-pressure-relating information obtained by the blood-pressure-relating-information obtaining device at the second time, and the corrected-blood-pressure determining means determines, based on the change value determined by the blood-pressure-change-value determining means, one of the first and second blood pressures to a corrected blood pressure that would have been measured at one of the first and second times that corresponds to the other of the first and second blood pressures. And, the blood-pressure-index determining means determines the superior-and-inferior-limb blood-pressure index, based on the corrected blood pressure determined by the corrected-blood-pressure determining means and the other of the first and second blood pressures that has not been corrected by the corrected-blood-pressure determining means. Since the superior-and-inferior-limb blood-pressure index is determined based on the two blood-pressure values measured at the same time, it enjoys a high accuracy.
Preferably, the blood-pressure-relating-information obtaining device comprises a pulse-wave-propagation-velocity-relating-information obtaining device which iteratively obtains, as the piece of blood-pressure-relating information, a piece of pulse-wave-propagation-velocity-relating information relating to a velocity at which a pulse wave propagates through an artery of the subject.
Since the pulse-wave-propagation-velocity-relating information is one of those sorts of blood-pressure-relating information that change most faithfully corresponding to the change of blood pressure, each of the change value determined by the blood-pressure-change-value determining means, the corrected blood pressure obtained based on the change value, and the superior-and-inferior-limb blood-pressure index determined based on the corrected blood pressure enjoys a high accuracy.
Preferably, the first blood-pressure measuring device includes a right-ankle first inflatable cuff adapted to be wound around a right ankle of the subject, and a left-ankle first inflatable cuff adapted to be wound around a left ankle of the subject, and measures a right-ankle first blood pressure of the right ankle and a left-ankle first blood pressure of the left ankle, the blood-pressure-change-value determining means determines a right-ankle first change value of the blood pressure of the subject between a right-ankle first time when the right-ankle first blood pressure is measured by the first blood-pressure measuring device and the second time when the second blood pressure is measured by the second blood-pressure measuring device, based on a right-ankle first piece of blood-pressure-relating information obtained by the blood-pressure-relating-information obtaining device at the right-ankle first time and the second piece of blood-pressure-relating information obtained by the blood-pressure-relating-information obtaining device at the second time, and additionally determines a left-ankle first change value of the blood pressure of the subject between a left-ankle first time when the left-ankle first blood pressure is measured by the first blood-pressure measuring device and the second time, based on a left-ankle first piece of blood-pressure-relating information obtained by the blood-pressure-relating-information obtaining device at the left-ankle first time and the second piece of blood-pressure-relating information obtained by the blood-pressure-relating-information obtaining device at the second time, the corrected-blood-pressure determining means corrects, based on the right-ankle change value determined by the blood-pressure-change-value determining means, one of the right-ankle first blood pressure and the second blood pressure to a corrected blood pressure that would have been measured at one of the right-ankle first time and the second time that corresponds to the other of the right-ankle first blood pressure and the second blood pressure, and additionally corrects, based on the left-ankle change value determined by the blood-pressure-change-value determining means, one of the left-ankle first blood pressure and the second blood pressure to a corrected blood pressure that would have been measured at one of the left-ankle first time and the second time that corresponds to the other of the left-ankle first blood pressure and the second blood pressure, and the blood-pressure-index determining means determines a right-side blood-pressure index, based on the corrected blood pressure determined by the corrected-blood-pressure determining means and the other of the right-ankle first blood pressure and the second blood pressure that has not been corrected by the corrected-blood-pressure determining means, and additionally determines a left-side blood-pressure index, based on the corrected blood pressure determined by the corrected-blood-pressure determining means and said other of the left-ankle first blood pressure and the second blood pressure that has not been corrected by the corrected-blood-pressure determining means.
According to this feature, the blood-pressure-index determining means determines both the right-side blood-pressure index and the left-side blood-pressure index. Since the lower one of the two blood-pressure index values can be used to make a diagnosis on the subject, the diagnosis enjoys a higher accuracy. In some cases, a blood pressure of an ankle may not be accurately measured because a tibia or a fibula prevents a posterior tibial artery from being sufficiently pressed, that is, a higher blood pressure than an accurate blood pressure may be measured. In those cases, an accurate diagnosis may not be expected. However, according to this feature, at least one of the right-side blood-pressure index and the left-side blood-pressure index is expected to be accurate. Thus, a more accurate diagnosis may be made based on the lower one of the two blood-pressure index values.